Field of the Invention
The present invention relates to liquid epoxy resins and liquid epoxy resin blends that exhibit a reduced tendency to crystallize i.e. crystallization resistance. The reduced crystallization tendency liquid epoxy resin, liquid epoxy resin blends, and their thermoset formulations of the present invention are useful, for example, in coatings, adhesives and composites applications.
Description of Background and Related Art
Liquid epoxy resins (LER), for example LERs based on diglycidyl ether of bisphenol-A, are important raw materials for coatings, adhesives and composites applications. However, these LERs exhibit a high tendency to crystallize at lower temperatures of about 0° C. to ambient temperature (about 25° C.). Crystallization poses a handling problem for some customers using the LERs. It would therefore be an advantage to eliminate, or at least minimize, the crystallization tendency of liquid epoxy resins and liquid epoxy resin blends. The tendency of LER to crystallize depends on several factors including for example purity, composition, additives, water content of the LER; and some external factors such as storage history and temperature.
“Tendency to crystallize” or “crystallization tendency” is an intrinsic property of liquid epoxy resins that, under certain conditions, the liquid epoxy resins crystallizes. “Crystallization” may take the form of turbidity or of crystals in the resin or on the container housing the resin. Occasionally, crystallization may continue to the point where complete solidification of the resin occurs. This solidification occurrence is not an indication that an epoxy resin is contaminated or out of specification. Rather, crystallization is a phenomenon which can occasionally occur during storage at room temperature (about 25° C.) or below. A LER exhibiting a “reduced crystallization tendency” means that crystallization of the LER is delayed or eliminated when compared with a conventional liquid epoxy resin and liquid epoxy resin blend.
Heretofore, one recommendation to prevent LER crystallization has been to properly store liquid epoxy resins, particularly at moderate to higher temperatures for an extended period of time. Storage of LER at temperatures above 55° C. for an extended period of time may provide an LER that is satisfactory for processing purposes, however, at these high temperatures, some color develops in the LER. Storage at 55° C. will reduce viscosity as well as prevent crystallization. Storage at temperatures at 55° C. to 75° C. will have some negative effect on color. It is not always possible to store a resin in a warmer or temperature controlled environment. Epoxy resin samples already in store shelves, stored in warehouses, or in transit are prone to crystallization at lower temperatures.
There are other methods that have been used heretofore to minimize or prevent the crystallization of LER. For example, LER crystallization tendency can be decreased by (1) blending the LER with diluents like cresyl diglycidyl ether (CGE) and p-t-butylphenyl glycidyl ether; (2) blending LER with another epoxy resin such as D.E.R. 354 (bisphenol F epoxy resin); or (3) modifying LER such as by esterifying LER with a long chain monocarboxylic acid such as a tall oil fatty acid, lauric acid, or linseed oil. The effectiveness of the above solutions varies in reducing crystallization tendencies and some of the above solutions are effective at the expense of higher cost and/or poorer performance. Therefore, the work used heretofore to reduce or prevent the crystallization of LER still needs further improvement.
There is definitely a need for a low cost reduced crystallization tendency LER with minimal negative performance properties of the final coating made from the LER. It would advance the industry if the crystallization tendency of LERs and/or liquid epoxy resin blends could be reduced without sacrificing any of the beneficial properties of the LER and/or the final thermoset made from the LER. Therefore, it is desired to provide a liquid epoxy resin with reduced crystallization tendency which not only achieves a sufficient crystallization resistance but also does not impact the final LER performance properties and/or cost.